Minimally invasive medical procedures often employ instruments that are controlled with the aid of a computer or through a computer interface. FIG. 1, for example, shows a robotically controlled instrument 100 having a structure that is simplified to illustrate basic working principles of some current robotically controlled medical instruments. (As used herein, the terms “robot” or “robotically” and the like include teleoperation or telerobotic aspects.) Instrument 100 includes a tool or end effector 110 at the distal end of an elongated shaft or main tube 120. In the illustrated example, end effector 110 is a jawed tool such as forceps or scissors having separate jaws 112 and 114, and at least jaw 112 is movable to open or close relative to jaw 114. In use during a medical procedure, end effector 110 on the distal end of main tube 120 may be inserted through a small incision in a patient and positioned at a work site within the patient. Jaws 112 may then be opened and closed, for example, during performance of surgical tasks, and accordingly must be precisely controlled to perform only the desired movements. A practical medical instrument will, in general, require many degrees of freedom of movement in addition to opening and closing of jaws 112 and 114 in order to perform a medical procedure.
The proximal end of main tube 120 attaches to a transmission or drive mechanism 130 that is sometimes referred to as backend mechanism 130. Tendons 122 and 124, which may be stranded cables, rods, tubes, or combinations of such structures, run from backend mechanism 130 through main tube 120 and attach to end effector 110. A typical surgical instrument would also include additional tendons (not shown) that connect backend mechanism 130 to other actuated members of end effector 110, a wrist mechanism (not shown), or actuated vertebrae in main tube 120, so that backend mechanism 130 can manipulate the tendons to operate end effector 110 and/or other actuated elements of instrument 100. FIG. 1 illustrates jaw 112 as having a pin joint structure 116 that provides a single degree of freedom for movement of jaw 112. Two tendons 122 and 124 are attached to jaw 112 and to a pulley 132 in backend mechanism 130, so that rotations of pulley 132 cause jaw 112 to rotate.
Pulley 132 is attached to a drive motor 140, which may be at the end of a mechanical arm (not shown), and a control system 150 electrically controls drive motor 140. Control system 150 generally includes a computing system along with suitable software, firmware, and peripheral hardware. Among other functions, control system 150 generally provides a surgeon or other system operator with an image (e.g., a stereoscopic view) of the work site and end effector 110 and provides a control device or manipulator that the surgeon can operate to control the movement of end effector 110. The software or firmware needed for interpretation of user manipulations of the control device and for generation of the motor signals that cause the corresponding movement of jaw 112 are generally complex in a real robotic medical instrument. To consider one part of the control task, the generation of the control signals for drive motor 140 commonly employs the relationship between the angle or position of jaw 112 and the angle or position of drive motor 140 or pulley 132 in backend mechanism 130. If the tendons 122 and 124 are rigid (e.g., if stretching of tendons is negligible), control system 150 can use a direct relationship between the angular position of drive motor 140 and the angular position of jaw 112 as defined by the geometry of instrument 100 in determining the control signals needed to move jaw 112 as a surgeon directs. Minor stretching of tendons 122 and 124, for example, under a working load, can be handled by some mathematical models relating motor position to effector position. However, if the mechanical structure including end effector 110, tendons 122 and 124, and backend mechanism 130 has a high degree of compliance, a relationship between the angular position of motor 140 (or pulley 132) and the angular position of jaw 112 may be difficult or impossible to model with sufficient accuracy for a medical instrument. Accordingly, such systems require control processes that do not rely on a fixed relationship between the applied actuator control signals and the position of the actuated elements.
It should be noted that in the following, the joint of the medical instrument can be a pin joint structure or a structure that provides one or more degrees of freedom of motion to the instrument tip. For instance a joint can be a continuously flexible section or a combination of pin joints that approximates a continuously flexible section or a single rotary joint that is not purely revolute but provides also some rolling joint. See, for example, U.S. Pat. No. 7,320,700, by Cooper et Al., entitled “Flexible Wrist for Surgical Tool,” and U.S. Pat. No. 6,817,974, by Cooper et Al., entitled “Surgical Tool Having a Positively Positionable Tendon-Actuated Multi-disk Wrist Joint.”
It should also be noted that in the state of the art of control of medical robotic instruments, the actuator positions are servo controlled to produce the desired instrument tip motion or position. Such an approach is effective as long as the transmission systems between the actuators and the instrument joints are rigid for all practical purposes. See, for example, U.S. Pat. No. 6,424,885, entitled “Camera Referenced Control in a Minimally Invasive Surgical Apparatus.” Such an approach can also be effective if the flexibility of the transmission system can be modeled exactly and a model included in the controller as described in U.S. Pat. App. Pub. No. 2009/0012533 A1, entitled “Robotic Instrument Control System” by Barbagli et Al.